U.S. Pat. No. 5,233,911 discloses a typical tandem brake booster through which a force is developed to effect a brake application. In such a brake booster, the interior is divided into first and second front chambers and first and second rear chambers by corresponding first and second walls. The first front chamber is externally connected to a first source of fluid pressure by a conduit and internally directly connected to the second front chamber and to the first and second rear chambers by various passages in a hub associated with the first and second wall. A control valve located in the hub is positioned such that the first fluid pressure is communicated in the first and second rear chambers to define a rest or ready position. In responsive to an input force, the control valve is positioned such that a second fluid pressure is presented to the first and second rear chambers to create a pressure differential across the first and second walls. This pressure differential acts on the first and second walls to develop an output force to assist in effecting a brake application. On termination of the input force, a spring returns the control valve to a rest or ready position of the first mode of operation as the second fluid is evacuated from the first and second rear chambers to allow the first fluid pressure to again be present in the first and second rear chambers. This type vacuum brake booster functions in an adequate manner to provide an assist in effecting a brake application.
As long as the operational fluid is communicated to the front chamber such vacuum brake boosters function to provide an assist in effecting a brake application, however, should a source of operational fluid be unavailable as when a vehicle is not operating or should a leak occur in the communication conduit the ability to create a pressure differential is diminished or terminated. In order to overcome such occurrences and still be able to effect a brake application, an internal accumulator chamber has been disclosed in U.S. Pat. No. 4,453,450 to provide a reserve source of operational fluid. However the increase in weight for the housing and structural complexity for the valving associated with the operation of the reserve source of operational fluid for such a vacuum brake booster has not been a commercial accepted product.
The same lack of commercial success could be attributed to a tandem brake booster disclosed in U.S. Pat. No. 3,813,992 where two single brake boosters are sequentially joining together. Additionally. the space required to accommodate such a dual arrangement have not been available with respect to most currently manufactured vehicles.